supercooled liquids

In a pure liquid, molecules touch one another but change neighbors frequently.  External forces cause the liquid to change shape by viscous flow.  Thermal agitation causes  molecules to undergo self-diffusion.  The two phenomena--viscous flow and self-diffusion--often result from a single rate-limiting process:  molecules change neighbors.  This simple picture is amply confirmed by the Stokes-Einstein relation, which links the viscosity and self-diffusivity for many liquids over wide ranges of temperature.